In order to increase power generation efficiencies of solar cells, conventionally, a method in which ruggedness is formed on the surface of a semiconductor substrate for solar cells, and incident light from the substrate surface is efficiently incorporated into an internal of the substrate has been employed. As a method of uniformly forming fine ruggedness on a substrate surface, a method including subjecting a (100) plane of a monocrystalline silicon substrate with an anisotropoic etching treatment using a mixed aqueous solution of sodium hydroxide and isopropyl alcohol, to form ruggedness of pyramidal forms (quadrangular pyramidal form) constituted by (111) plane has been known. However, since isopropyl alcohol (IPA) is used, this method has some disadvantages in the aspect of fluctuations of quality due to fluctuations of components due to evaporation of IPA or waste liquid treatment, operating environment, and safety. In addition, the treatments at relative high temperatures for a long amount of time are needed, so that improvements are desired in the aspect of productivity.
In the techniques of improving the above, Patent Publication 1 describes a method in which etching rates when the substrate surface is etched are stabilized by including a specified aliphatic carboxylic acid and silicon in an alkaline an etching agent, thereby uniformly forming pyramid-shaped ruggedness of desired sizes on the substrate surface.
On the other hand, in a case of a polycrystalline silicon substrate, since plural crystal orientation planes appear on the surface, sufficient lowering of the reflectivities is not seen by the above alkali type etching treatment. Therefore, generally, etching is carried out using a mixed acid of hydrofluoric acid and nitric acid, to form ruggedness on the surface.